In the field of oral formulation development, a fundamental challenge persists: how to maintain the stability of acid-sensitive Active Pharmaceutical Ingredients (APIs) in the low-pH gastric environment? A large number of APIs undergo degradation, structural changes or premature release in gastric juice before reaching the intestinal absorption site, directly leading to reduced bioavailability and unstable therapeutic effects. This issue is transforming enteric-coated capsules from a traditional "dosage form option" into a "necessary formulation solution" to ensure drug efficacy.

Clinical Dilemmas of Acid-Sensitive Drugs

Gastric juice has a pH value typically ranging from 1.0 to 3.5 and contains strong acidic components such as pepsin. This environment is lethal to many drug molecules. Proton Pump Inhibitors (PPIs, e.g., omeprazole, lansoprazole, pantoprazole sodium), certain polypeptide drugs, macrolide antibiotics, and plant-derived APIs are highly susceptible to hydrolysis, rearrangement or structural collapse in acidic conditions.

Direct exposure of acid-sensitive drugs to gastric acid often results in the following clinical issues:

• Insufficient effective dosage due to partial API degradation before reaching the absorption site
• Unstable release behavior, leading to unpredictable in vivo pharmacokinetics
• Significant batch-to-batch variation in bioavailability, affecting the consistency of clinical efficacy
• Difficulty in controlling formulation stability, increasing product quality risks

Research indicates that in many cases, the problem lies not in inadequate intestinal absorption capacity, but in partial drug inactivation before it reaches the intestines. This shift in understanding has led formulation developers to refocus their efforts from "how to enhance absorption" to "how to protect drugs prior to absorption".

Core Value of Enteric-Coated Capsules: Gastric Protection Rather Than Simple Delayed Release

The industry’s understanding of enteric coating technology is undergoing profound changes. For acid-sensitive drugs, the most important function of enteric-coated capsules is not to delay release, but to protect APIs from destruction in the stomach.

By remaining intact in the stomach, enteric-coated capsules form a robust physical barrier, ensuring that drugs enter the nearly neutral intestinal environment (pH 5.5-7.5) in their original form. This "site-specific release" mechanism enables drugs to be released in their intact form at the optimal absorption site, thereby maximizing bioavailability.

Clinical Value: From Drug Protection to Efficacy Assurance

Avoiding the Risk of Gastric Degradation

Take PPIs as an example—these drugs are extremely unstable in acidic environments. A well-designed enteric coating process can ensure that drugs are not released in gastric juice but rapidly dissolve in the intestines. For instance, research on the lansoprazole enteric-coated pellet system shows that its release rate can reach over 94% within 8 hours after entering the intestines, guaranteeing the absorption of the drug in its original form.

Protecting Bioactive Substances

Enteric protection is equally crucial for biologically derived active ingredients. Studies have confirmed that enteric-coated capsules can effectively protect the biological activity of extracts from damage by gastric acid and pepsin; after 2 hours of exposure to a simulated gastric fluid environment, the contents still retain significant biological activity.

Improving the Consistency of Bioavailability

By shifting the release starting point from the stomach to the small intestine, enteric-coated capsules greatly reduce the interference of the complex physiological gastric environment (e.g., feeding status, gastric emptying rate, individual differences in gastric acid secretion) on drug release. This characteristic significantly lowers the coefficient of variation in in vivo absorption, improves the reliability of clinical efficacy, and also simplifies the risk control points in the Quality by Design (QbD) process.

Industrial Trend: Why Enteric Coating Technology Is Becoming Increasingly Important

With the development of formulation technology, the demand for enteric-coated capsules is showing a significant growth trend, mainly in the following fields:

• Acid-sensitive APIs: Including PPIs, certain antibiotics, enzyme preparations, etc.
• Plant-derived active ingredients: Many natural products have poor stability in acidic environments
• Probiotics and Fecal Microbiota Transplantation (FMT) preparations: Requiring protection of viable bacteria from being killed by gastric acid
• Colon-targeted preparations: Needing to ensure that drugs pass through the stomach and small intestine intact and release at the colonic site

For these products, enteric-coated capsules are no longer just an "optimization option", but a key tool to ensure drug efficacy.

Key Considerations for Formulation Development

Enteric Coating Technology

The efficacy of enteric coating depends on the thickness and integrity of the coating layer. Insufficient coating thickness or defects such as pores and cracks may lead to premature drug release in the stomach, affecting drug efficacy and patient safety. Research shows that for some commonly used enteric coating materials, the critical thickness to ensure effective acid protection is typically between 65 and 69 micrometers.

Quality Control Requirements

In accordance with General Rule 0921 Disintegration Time Limit Test in Volume IV of the Pharmacopoeia of the People's Republic of China (2020 Edition), the disintegration test for enteric preparations is conducted in two stages: the first stage in 0.1mol/L hydrochloric acid solution, requiring the capsules to show no disintegration, cracking or softening within 2 hours; the second stage in phosphate buffer solution (pH 6.8) to verify their release characteristics in the intestinal environment, requiring complete disintegration within 1 hour.

The value of enteric-coated capsules for acid-sensitive drugs has evolved from "selective modified release" to a "necessary means to guarantee basic efficacy". It solves the problem of gastric acid "intercepting and deactivating" drug molecules and ensures the stability of the entire chain from formulation production to in vivo absorption. As more acid-sensitive molecules enter the research pipeline, enteric-coated capsules are becoming an indispensable technical carrier in modern oral formulation development. For formulation engineers and pharmaceutical enterprises, an in-depth understanding and rational application of enteric coating technology will play an increasingly important role in ensuring product quality and enhancing clinical value.

References

[1] Chinese Pharmacopoeia Commission. Pharmacopoeia of the People's Republic of China (2020 Edition) Volume IV [S]. Beijing: China Medical Science Press, 2020: General Rule 0921.